System and method for transmission-line termination by signal cancellation, and applications thereof

ABSTRACT

An active terminating device ( 30 ) for an electrical transmission line with optional line-receiving and line-driving capabilities. The basic device is a two-terminal unit, denoted as a Signal Canceling Unit (SCU), which senses the signal available at its terminals ( 34   a,    34   b ), and applies negative feedback in order to cancel and absorb the signal. When applied to the end of a transmission line ( 15   a,    15   b ) as part of wired communication network, the SCU functions as a terminator. When connected in the middle of such wired transmission line, the SCU splits the transmission line into two separate and isolated segments. In such a configuration, the SCU can be used to isolate a portion of a network from signal degradation due to noise or bridge-tap. Furthermore, the two isolated segments may each employ independent communications, such that no interference exists between the segments. In another embodiment, line receiver functionality is integrated into the SCU, designated as a Signal Canceling and Receiving Unit (SCRU) ( 90 ). The SCRU can perform all the SCU functions, and also serves as a line receiver in the communication network. In yet another embodiment, line driver functionality is integrated into the SCRU, designated as a Signal Canceling, Receiving and Transmitting Unit (SCRTU) ( 120 ). The SCRTU can perform all the SCRU functions, and also serves as a line driver in the communication network. Upon connecting multiple SCRTU&#39;s to a continuous transmission line, terminated independent point-to-point communication segments are formed.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of electrically-wiredcommunication, and, in particular, to communication lines employingtermination.

BACKGROUND OF THE INVENTION

[0002] The term “data unit” herein denotes any data processing device,such as a computer or a personal computer, including workstations orother data terminal equipment (DTE) with an interface for connection toany wired communication network, such as a Local Area Network (LAN).

[0003] Transmission lines over which digital signals are transmittedmust be properly terminated in order to prevent overshoot, undershootand reflections. These effects, when caused by impedance mismatch,become more pronounced as the length of the conductor increases, andlimit the rate at which data can be transmitted over a transmissionline. The transmission line can be a trace on an integrated circuit, atrace on a board, or a wire in a cable. The impedance of both the sourceand load should be matched to the characteristic impedance of thetransmission line. Since the output impedance of a transmitter and theinput impedance of a receiver generally differ from the characteristicimpedance of a transmission line interconnecting the transmitter and thereceiver in a point-to-point configuration, it is necessary to alter theexisting impedance differently at the source and load ends of thetransmission line.

[0004] Wire-based communication networks commonly employ terminations inorder to avoid reflections. An example of termination within a networkis shown in FIG. 1. A shared wired network 10 is based on a two-wiretransmission line having wires 15 a and 15 b. In the followingdescription, reference will be made to “transmission line 15 a and 15b”, it being understood that the reference numerals actually refer tothe wires forming the transmission line. For example, network 10 may bean EIA/TIA-485 standard type, wherein transmission line 15 a and 15 bconsists of a single twisted pair, or an Ethernet IEEE802.3 standard10Base2 or 10Base5, wherein transmission line 15 a and 15 b is a coaxialcable. In general, the term ‘transmission line’herein denotes anyelectrically-conductive media capable of carrying electrical current andvoltages, and transporting electromagnetic signals, including withoutlimitation wires, cables, and PCB traces. Differential line drivers 11 aand 11 bare used in order to transmit signals to the transmission line,while line-receivers 12 a and 12 b are used to receive signals carriedover transmission line 15 a and 15 b. Data unit 16 a is a “transmitonly” unit, which transmits data to the transmission line via linedriver 11 a, and data unit 16 b is a “receive only” unit that receivesdata from the transmission line via line receiver 12 a. Data unit 16 ccan both receive data from and transmit data to the transmission line 15a and 15 b via line diver 11 b and line receiver 12 b, forming atransceiver 14. Of course, additional units can be connected to sharedtransmission lines, each such units employing a line receiver, a linedriver, or both. In order to allow for proper operation of network 10,terminators 13 a and 13 b are commonly installed and connected to bothends of transmission line 15 a and 15 b. In order to function properly,terminators 13 a and 13 b should be equal in impedance to thecharacteristic impedance of transmission line 15 a and 15 b. Similarly,such terminations are employed in both ends of a point-to-pointconnection.

[0005] The need for termination is a major drawback in building anetwork. First, the transmission line ends must be identified andaccessed, which may not be simple in the case of existing wiring.Additionally, terminator installation requires both labor and materials,and there is also the issue of additional equipment required toconfigure a network. Furthermore, for proper operation, the terminationtype, topology and values are mainly based on the transmission linecharacteristics, which may be unknown and/or inconsistent, and may varyfrom cable to cable or from location to location.

[0006] An additional drawback of network 10 relates to being amulti-point shared transmission line network. In a Time DomainMultiplexing (TDM) scheme, only a single driver can transmit over thetransmission line during any time interval, rendering other units asreceive-only during that time interval. This limits the total volume ofdata that can be transported over a specified period. In order to allowmultiple data transport over this shared transmission line, it isnecessary to allow multiple transmitters and receiver to use thetransmission line simultaneously.

[0007] One common method for such multiple transmissions over sharedtransmission line employs the Frequency Domain Multiplexing (FDM)scheme, wherein each transmitter uses a different dedicated portion ofthe transmission line's available spectrum. Such a solution, however,requires complex and expensive circuitry.

[0008] Another method for enabling multiple transmissions is shown inFIG. 2, and involves splitting the transmission line into distinctsegments. A network 20 is shown in part, wherein the transmission lineis split into two distinct portions, one of which is identified astransmission line segment 15 a and 15 b (as in FIG. 1), while the otherportion is identified as a transmission line segment 15 c and 15 d.Transmission line segment 15 a and 15 b is used for full duplexcommunication using line drivers 11 a 2 and 11 b 1, located atrespective ends of transmission line segment 15 a and 15 b. Similarly,line receivers 12 b and 12 a 2 as well as terminators (not shown) areinstalled at the respective ends of transmission line segment 15 a and15 b. Line driver 11 a 2 and line receiver 12 a 2 are both part of aunit 21 a, which is connected at one end of transmission line segment 15a and 15 b. Similarly, transmission line segment 15 c and 15 d iscoupled to line drivers 11 c 1 and 11 b 2, as well as to line receivers12 c 1 and 12 b 2. Line driver 11 c 1 and line receiver 12 c 1 are bothpart of a unit 21 c, connected at one end of transmission line segment15 c and 15 d. Line drivers 11 b 2 and 11 b 1, as well as line receivers12 b 1 and 12 b 2 are all part of a unit 21 b, connected to transmissionline segment 15 a and 15 b, and to transmission line segment 15 c and 15d. These two distinct transmission line segments as well as theirrelated drivers/receivers are coupled by a logic block 22, which is partof unit 21 b. In certain prior art configurations, the logic block iseither omitted or acts as transparent connection. In such case, unit 21b serves as a repeater. In other configurations, logic block 22processes the data streams flowing through unit 21 b.

[0009] Network 20 offers two major advantages over network 10 as shownin FIG. 1. First, each transmission line segment of network 20 isindependent, allowing two communication links to operate simultaneously.Hence, line driver 11 a 2 of unit 21 a can transmit data overtransmission line segment 15 a and 15 b, to be received by line receiver12 b 1 of unit 21 b. Simultaneously, and without any interference, linedriver 11 c 1 of unit 21 c can transmit data over transmission linesegment 15 c and 15 d to be received by line receiver 12 b 2 of unit 21b.

[0010] Yet another advantage of network 20 is that of havingpoint-to-point communication segments. As is well known in the art,point-to-point topology is a highly favored configuration in wiredcommunication, enabling robust, high bandwidth communications withlow-cost, simple circuitry.

[0011] Principles of the above description are demonstrated by theevolution of the Ethernet Local Area Network (LAN) as specified in theIEEE802.3 standard, wherein shared transmission line systems based oncoaxial cable 10Base2 and 10Base5 were upgraded towards 10BaseT and10BaseTX based networks, both built around point-to-point segments.

[0012] However, network 20 also exhibits a major disadvantage incomparison to network 10. As shown in FIG. 1, network 10 uses acontinuous uninterrupted transmission line. In contrast, the wiring ofnetwork 20 must be cut at several points throughout the network, whereinunits 21 are simply connected. In the case of existing transmissionlines (such as in-wall telephone wiring), cutting into the network maybe complex, expensive, and labor-intensive.

[0013] There is thus a widely recognized need for, and it would behighly advantageous to have, a means for implementing a generictermination that is not transmission line-dependent, and which thereforewould not need to be changed when the transmission line characteristicschange. There is also a widely recognized need for a means forsimultaneous multiple use of a single wiring infrastructure, and foremploying a point-to-point connection scheme, without modifying suchexisting wiring. These goals are addressed by the present invention.

SUMMARY OF THE INVENTION

[0014] The invention relates to a system and method for signaltermination, based on a two-port unit, denoted herein as a SignalCanceling Unit (SCU). The SCU senses the signal present over itsterminal, and operates to absorb and cancel this signal. When connectedat an end of a transmission line, such as a wire transmission line usedfor communication, the SCU functions as a terminator by absorbing thesignal energy. When connected in the middle of such wiring transmissionline, the SCU terminates any signal sensed over its terminals, and thuscan be used for noise isolation, or to emulate a network end in theconnected points. In this functional mode, the SCU effectively splitsthe wires, allowing for different independent networks operation at eachside of the SCU connection, without interfering or interacting with eachother, even though the continuity of the wiring is not affected.

[0015] In another embodiment, the SCU is upgraded to include linereceiver functionality, denoted herein as a Signal Canceling andReceiving Unit (SCRU). In addition to having full SCU functionality, theSCRU also operates as a line receiver, and hence can be used as anactive receiver in the network, in addition to serving in terminationand signal canceling roles.

[0016] In yet another embodiment, the SCRU is upgraded to include linedriver functionality, denoted herein as a Signal Canceling, Receiving,and Transmitting Unit (SCRTU). In addition having full SCRUfunctionality, the SCRTU also performs as a line driver, and hence canbe used as an active transmitter in the network, in addition to servingin termination, signal canceling, and receiving roles. Multiple SCRTU'sconnected to wired transmission lines can communicate for constructionof a full network. In such a network, every pair of adjacent-connectedSCRTUs can communicate in a point-to-point fashion, in a terminated andindependent transmission line segment.

[0017] Therefore, according to a broad aspect of the present inventionthere is provided a device for actively terminating and isolating acontinuously conducting transmission line, said device comprising:

[0018] a sensor operative to sensing a first signal on the transmissionline;

[0019] a first driver operative to placing a second signal on thetransmission line for canceling the first signal; and

[0020] a processing unit operative to receiving input from said sensorand providing input to said first driver.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to understand the invention and to see how it may becarried out in practice, a preferred embodiment will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

[0022]FIG. 1 shows a common prior art shared wired Local Area Networkconfiguration;

[0023]FIG. 2 shows a prior art repeater based communication network;

[0024]FIG. 3 shows a Signal Canceling Unit (SCU) functional blockdiagram according to a first embodiment of the present invention;

[0025]FIG. 4 shows a shared wiring based network, wherein an SCU is usedas an end terminator according to the present invention;

[0026]FIG. 5 shows a shared wiring based network, wherein an SCU is usedas a parallel connected terminator according to the present invention;

[0027]FIG. 6 shows a shared wiring based network, wherein an SCU is usedfor noise isolating according to the present invention;

[0028]FIG. 7 shows a shared wiring based network, wherein an SCU is usedfor bridge-tap isolating according to the present invention;

[0029]FIG. 8 shows a shared wiring based network, wherein an SCU is usedfor allowing multiple independent communication segments over continuouswiring according to the present invention;

[0030]FIG. 9 shows a Signal Canceling and Receiving Unit (SCRU)functional block diagram according to a second embodiment of the presentinvention;

[0031]FIG. 10 shows a shared wiring based network, wherein an SCRU isused for allowing multiple independent communication segments overcontinuous wiring according to the present invention;

[0032]FIG. 11 shows a Signal Canceling, Receiving and Transmitting Unit(SCRTU) functional block diagram according to a third embodiment of thepresent invention;

[0033]FIG. 12 shows an alternative Signal Canceling, Receiving andTransmitting Unit (SCRTU) functional block diagram according to a fourthembodiment of the present invention; and

[0034]FIG. 13 shows a shared wiring based network, wherein multipleSCRTU's are used for allowing multiple independent communicationsegments over continuous wiring according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0035] The principles and operation of a network according to thepresent invention may be understood with reference to the drawings andthe accompanying description. The drawings and descriptions areconceptual only. In actual practice, a single component can implementone or more functions; alternatively, each function can be implementedby a plurality of components and circuits. In the drawings anddescriptions, identical reference numerals indicate those componentsthat are common to different embodiments or configurations.

[0036]FIG. 3 illustrates a Signal Canceling Unit (SCU) 30, whichincludes two external terminal connections, a terminal 34 a (A) and aterminal 34 b (B). Coupled to these terminals is a sensor 31, whichmeasures the differential voltage (constituting a “first signal”)between terminal 34 a and terminal 34 b. The value measured by sensor 31is input into a processing unit 33, which in turn provides input to adifferential driver 32 (constituting a “first driver”), whose outputsare coupled to the terminal 34 a and terminal 34 b. Driver 32 can sinkor source enough current (constituting a “second signal”) to cancel thefirst signal at the terminals. Processing unit 33 along with sensor 31and driver 32 forms a closed negative feedback loop, which attenuatesand cancels any signal sensed over terminal 34 a and terminal 34 b.

[0037]FIG. 4 illustrates a network 40, with SCU 30 used as a terminator.Network 40 is based on network 10 (FIG. 1), but modified to use SCU 30as a terminator in place of terminator 13 b. Signals transmitted totransmission line 15 a and 15 b (by line driver 11 a, for example)propagate along the transmission line. Upon reaching the end of thetransmission line, where terminals 51 a and 51 b of SCU 30 areconnected, SCU 30 senses and acts to cancel the signals. As a result,the signal energy is absorbed by SCU 30, and neither reflection nor anyother mismatch occurs. Hence, SCU 30 acts as a termination device.However, since the structure of SCU 30 is generic and is not tailored tothe specific transmission line (e.g., characteristic impedance), thissame SCU can be used for many types of transmission line, such astwisted pair wiring, coaxial cables, etc., obviating the need to match aspecific termination to a specific transmission line. This, of course,provides simple installation and easy logistics, due to the employmentof common components for various different applications.

[0038] A further advantage of using an SCU as a terminator stems fromthe fact that the SCU performs the termination function even when notconnected at the ends of the transmission line, but at any pointthroughout the transmission line run, as illustrated in FIG. 5 for anetwork 50, which is based on transmission line 15 a, 15 b, 15 c, and 15d. As with network 10 (FIG. 1), terminator 13 is located at one end(left side of the figure), and line driver 11 a and line receivers 12 aand 12 b are coupled to the transmission line. Data units 16 a, 16 b,and 16 d are coupled to line units 11 a, 12 a, and 12 b, respectively.If SCU 30 were not present in network 50, network 10 of FIG. 1 would beobtained, wherein data unit 16 a can transmit data to the entiretransmission line via line driver 11 a. The transmitting signals wouldthen propagate in the transmission line and would be received by dataunits 16 b and 16 d via line receivers 12 a and 12 b, respectively. Inthis case, however, where SCU 30 is connected to the transmission lineat connection points 51 a and 51 b, the network 50 is modified such thatsignals transmitted to line driver 11 a, are propagated in thetransmission line in two directions. Part of the signal energy ispropagated towards terminator 13 (towards the left side of the figure),where they are absorbed. The other part of the signal energy propagatestowards points 51 c and 51 d, representing the other end of the wiring.When the signal reaches points 51 a and 51 b (connected to the terminalsof SCU 30), SCU 30 operates to attenuate, cancel, and absorb the signalenergy. Thus, little or no signal will propagate from the points 51 aand 51 b towards the end points 51 c and 51 d. In such case, while linereceiver 12 a will receive the transmitted signals, line receiver 12 bwill not sense any such signals, which are attenuated by SCU 30. Thus,SCU 30 functions as a terminator for the network segment 15 a and 15 b,extending from terminator 13 to points 51 a and 51 b, helping to avoidreflections in this part of the transmission line. As a result, SCU 30modifies the functionality of the continuous transmission line to bevirtually separated into two distinct segments, one using thetransmission line from terminator 13 to points 51 a and 51 b, while theother uses the transmission line from points 51 a and 51 b to theend-points 51 c and 51 d. The two network segments are isolated in thesense that signals in one segment cannot pass to the other, even thoughelectrical continuity of the transmission line is fully retained.

[0039] One application of such virtual networks separation is for noiseisolation, as illustrated in FIG. 6 with a network 60. Network 60 issimilar to network 50 (FIG. 5), except that a noise source 61 appears inplace of data unit 16 d and line receiver 12 b. The noise generated bynoise source 61 propagates (in the left direction) towards SCU 30. Uponreaching SCU terminals 51 a and 51 b, SCU 30 operates to attenuate thenoise signal, and prevents the noise from reaching transmission line 15a and 15 b and thereby degrading communication over that networksegment. While noise source 61 is described and illustrated as adistinct unit connected at a single point to transmission line 15 c and15 d, the same noise cancellation function is performed where noise isgenerated by inductive means from external sources. For example,transmission line 15 c and 15 d may extend over an area near sources ofelectromagnetic interference. The SCU can thus help in isolating theinduced noise from a specific portion of the conductive transmissionline.

[0040] Bridge-taps are known to cause impedance mismatch and reflectionsin transmission lines and other wired communication environments. FIG. 7illustrates a network 70, which is similar to network 60 (FIG. 6), butwith added transmission line 15 e and 15 f, connected to terminals 51 aand 51 b respectively, forming a bridge tap at terminals 51 a and 51 b.Without SCU 30, the bridge tap at these points would create an impedancemismatch and cause signal reflections in the communications overtransmission line 15 a, 15 b, 15 c, 15 d, 15 e, and 15 f. The presenceof SCU 30 at the bridge-tap junction, however, cancels and absorbs thesignals at terminals 51 a and 51 b, and eliminates such reflections. Indoing so, three isolated communication segments are formed, one segmentconsisting of transmission line 15 a and 15 b, a second segmentconsisting of transmission line 15 c and 15 d, and a third segmentconsisting of transmission line 15 e and 15 f.

[0041] The capability of an SCU to isolate electrically connectedtransmission line enables the formation of multiple distinctcommunication networks over continuous electrical conductingtransmission line, as shown in FIG. 8. A network 80 is based ontransmission line 15 a, 15 b, 15 c, and 15 d. SCU 30 connects to thetransmission line at terminals 51 a and 51 b, and isolates thetransmission line into two communication segments. One segment is basedon transmission line 15 a and 15 b, and extends from terminals 51 a and51 b towards the left in FIG. 8. The other segment is based ontransmission line 15 c and 15 d, and extends toward the right. Data unit16 a transmits across transmission line 15 a and 15 b via line driver 11a, and provides the signal received by data unit 16 b via line receiver12 a. Similarly, data unit 16 e transmits across transmission line 15 cand 15 d via line driver 11 b, with the signal received by data unit 16d via line receiver 12 b. Being isolated by SCU 30, both transmissionscan occur simultaneously, without interfering with each other.Additional line drivers, line receivers and transceivers can be added toeach communication segment. Similarly, adding additional SCU's can splitelectrically-connected transmission line into more segments, wherein anisolated segment is formed between adjacent SCU pairs, or between theSCU and the ends or terminators of transmission lines.

[0042] The function of the SCU has been so far been described only as aterminator, but an SCU can also be modified to perform a line receivingfunction, as shown in FIG. 9, which illustrates a Signal Canceling andReceiving Unit (SCRU) 90. SCRU 90 is based on the structure of SCU 30,(FIG. 3), but the processing unit 33 is modified to a processing unit91, which provides additional output via a terminal 34 c (C). The outputon terminal 34 c uses sensing function 31, and together with part ofprocessing unit 91 serves as a line receiver, similar to line receiver12 a or 12 b. Thus, SCRU 90 simultaneously performs two functions:signal cancellation as does SCU 30, and line receiver functionality, asdo line receivers 12 a and 12 b, thus allowing the sensed signal or anyfunction thereof to be output on the terminal 34 c and placed on thetransmission line.

[0043] An example of an application using SCRU 90 is shown in FIG. 10,for a network 100. Network 100 is based on network 80 (FIG. 8), but SCU30 is replaced by SCRU 90, whose terminal C is connected to a data unit16 fvia a connection 102. SCRU 90 further is connected to transmissionline 15 a, 15 b, 15 c, and 15 d at junctions 101 a and 101 b. In amanner similar to that of network 80 (FIG. 8), this configuration allowstwo isolated communication segments to use the transmission linesimultaneously without interfering with each other. One segmenttransports data over transmission line 15 a and 15 b, while the othersegment transports data over transmission line 15 c and 15 d. Inaddition, by utilizing the line-receiving functionality of SCRU 90, dataunit 16 f can receive signals from both networks.

[0044] In yet another embodiment of the invention, a line-drivingcapability is also integrated into the SCRU. FIG. 11 illustrates anSCRTU (Signal Canceling, Receive and Transmit unit) 110. SCRTU 110includes all components of SCRU 90, but also includes a line driver 111(constituting a “second driver”), which is fed from an additional SCRTUterminal 34 d (D) and feeds a third signal to the transmission line.SCRTU 110 has two states of operation, denoted as “receive” and“transmit”. In “receive” state, the functionality of SCRU 90 is fullyretained, and SCRTU 110 performs signal cancellation and reception. In“transmit” state, line terminals 34 a (A) and 34 b (B) are connected toline driver 111 output terminals as shown, so that SCRTU 110 cantransmit data received at terminal 34 d to terminals 34 a and 34 b.Shifting between the states is performed by two SPDT (single pole doublethrow) switches 112 and 113. Switches 113 and 112 are connected toterminals 34 a and 34 b, respectively. In the ‘receive’ state, bothswitches 112 and 113 are in state ‘1’, thus connecting terminal 34 a andterminal 34 b terminals to sensor 31 and driver 32, and therebyperforming the function of SCRU 90. In the ‘transmit’ state, bothswitches 112 and 113 are in state ‘2’, thus connecting terminal 34 a andterminal 34 b to the outputs of line driver 111, and thereby performingas a line driver. Switches 112 and 113 are controlled by a logic unit114, which changes switches 113 and 112 as required to select thedesired state.

[0045]FIG. 12 illustrates an alternative implementation of an SCRTU 120.In this alternative configuration, driver 32 is also used as the linedriver. An SPST switch 121 is used to route the input into driver 32. Instate ‘1’, driver 32 is connected to the output of processing 91, andthereby performing the function of SCRU 90. In state ‘2’, driver 32 iscoupled to terminal 34 d, and thereby functions as a line driver. Alogic block (not shown in FIG. 12) is used to control switch 121,shifting it from state to state as required.

[0046]FIG. 13 illustrates a network 130 using such SCRTU's. Network 130uses network transmission line 15 a, 15 b, 15 c, 15 d, 15 e, 15 f, 15 g,and 15 h, and has a bridge-tap at points 51 a and 51 b. Data units 16 f,16 g, 16 h, 16 i, and 16 j are coupled to the transmission line viaSCRTU's 110 a, 110 b, 110 c, 110 d, and 110 e, respectively. Asexplained above, although the wiring is electrically continuous, thecommunication segments formed are of point-to-point type between anySCRTU pair. SCRTU 110 a communicates in a point-to-point topology withSCRTU 11 b, over transmission line segment 15 a and 15 b. Similarly,SCRTU's 110 b and 110 e communicate over transmission line segment 15 eand 15 f, SCRTU's 110 b and 110 c communticate over transmission linesegment 15 c and 15 d, and SCRTU's 110 c and 110 d communicate overtransmission line segment 15 g and 15 h. In addition to the benefit ofpoint-to-point, the network also allows for multiple independentcommunication segments to operate independently, as long as there arenot any two SCRTU's transmitting to the same segment. For example, SCRTU110 a can transmit to SCRTU 110 b over transmission line segment 15 aand 15 b, while SCRTU 110 d can simultaneously transmit to SCRTU 110 cover transmission line segment 15 g and 15 h.

[0047] Network 130 demonstrates the SCRTU based network capability ofpoint-to-point communications and multiple transmissions over continuouswiring. These capabilities can be useful for existing wiring havingunknown topology, and having ‘bus’ type connection points. For example,in-wall existing telephone wiring, in-wall existing power lines or CATVcabling which are not used for their original purpose. Continuity iscommon to all of these types of wiring, where outlets are provided forconnecting to the wiring. Hence, coupling SCRTU's to each outlet allowsfor reliable high bandwidth communication between data units connectedto the SCRTU's.

[0048] While the invention has been described with respect to a digitalcommunication application, it will be appreciated that the invention isequally applicable to analog communication as well, such as video, audioor any other type of communication. In such configurations, data units16 are replaced by suitable analog units, and the SCU's, SCRU's, andSCRTU's are modified accordingly to support such communication.

[0049] While the invention has been described with respect to a limitednumber of embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

1. A device (30) for actively terminating and isolating a continuouslyconducting transmission line (15 a, 15 b), said device comprising: asensor (31) operative to sensing a first signal on the transmissionline; a first driver (32) operative to placing a second signal on thetransmission line for canceling the first signal; and a processing unit(33) operative to receiving input from said sensor and providing inputto said first driver.
 2. The device (90) as in claim 1 wherein theprocessing unit (90) is operative to receiving the first signal from thetransmission line, and further comprises an output (34 c) for outputtingsaid first signal or a function thereof.
 3. The device (120) as in claim2 further operative to transmitting second signals on the transmissionline, and furthermore comprising a switch (121) operative to selectivelyplacing the device in a state taken from a group including a receivestate and a transmit state.
 4. The device (110) as in claim 3,furthermore comprising: a second driver (111) operative to placing athird signal on the transmission line; at least one switch (112, 113)operatively coupled to the first and second drivers, and a logic unit(114) responsive to a first state (1) for controlling said switch (112,113) so as to feed the second signal to the transmission line therebycanceling the first signal and being responsive to a second state (2)for controlling said switch (112, 113) so as to feed the third signal tothe transmission line.
 5. A communications network (40) having anelectrical transmission line (15 a, 15 b) operative to carrying signalsand having a free end coupled to the device (30) according to any one ofclaims 1 to 4 for terminating the transmission line at the free end. 6.A communications network (50) having an electrically continuoustransmission line (15 a, 15 b) operative to carrying signals, thenetwork comprising a first segment (51 a, 51 b) and a second segment (51c, 51 d) coupled to the device (30) according to any one of claims 1 to4, for isolating signals in the first segment from the second segmentand for isolating signals in the second segment from the first segment.7. A communications network (130) having an electrically continuoustransmission line (15) operative to carrying signals, the networkcomprising a plurality of segments ((15 a,15 b), (15 c,15 d), (15 e,15f), (15 g,15 h)) each pair being coupled to a respective device (30)according to any one of claims 1 to 4, for isolating signals in each ofsaid segment from all other segments.
 8. A communications network (70)having an electrically continuous transmission line (15 a, 15 b)operative to carrying signals and having a bridge tap (51 a, 51 b)connected to the transmission line, the bridge tap also operative tocarrying signals, the network comprising the device (30) according toany one of claims 1 to 4 for isolating the signals of the bridge tapfrom the transmission line and for isolating the signals of thetransmission line from the bridge tap.
 9. A method for activelyterminating and isolating a continuously conducting transmission line(15 a, 15 b), said method comprising: (a) sensing a first signal on thetransmission line; (b) processing said first signal, and (c) placing asecond signal on the transmission line for canceling the first signal.10. The method as in claim 9, selectively canceling the first signal byplacing the second signal on the transmission line or by placing a thirdsignal on the transmission line.
 11. The method as in claim 9 or 10,further comprising the step of regenerating the canceled first signaland placing it on the transmission line.
 12. The method as in claim 9 or10, wherein the first signal is noise that is canceled by the secondsignal.